Lungfish

Lungfishes Fossil range: Early Devonian - Recent
Queensland Lungfish
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Sarcopterygii Subclass: Dipnoi Müller, 1844
Orders
See text.

Lungfish are sarcopterygian fish of the taxon Dipnoi, characterized by platelike teeth and lobed, paired fins, with modern forms typified by functional lungs, the caudal fin confluent with the dorsal and anal fins, and large scales. While some consider Dipnoi to be a subclass of Sarcopterygii, Nelson (2006) consider the subclass to be Dipnotetrapodomorpha, with the dipnoians (lungfishes) comprising two suborders within this subclass, Dipterimorpha and Ceratodontimorpha, the later including the living lungfishes. The dipnoans are well represented in the fossil record, but there are only six extant species, all freshwater species. The first living lungfish was formally described in 1837 (Nelson 2006).

Overview and description

Lungfish together with coelacanths comprise the vertebrate class Sarcopterygii. The bony fish representatives of this class, known as lobe-finned fish, are characterized by lobed paired fins, which are joined to the body by a single bone (Clack 2002). Some taxonomists also include the tetrapods (amphibians, reptiles, birds, mammals) in Sacropterygii in order to make this a monophyletic group.

Lungfish are best-known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed internal skeleton. Today, they live only in Africa, South America, and Australia. While vicariance would suggest this represents an ancient distribution limited to the Mesozoic supercontinent Gondwana, the fossil record suggests that advanced lungfish had a cosmopolitan freshwater distribution and that the current distribution of modern lungfish species reflects extinction of many lineages following the breakup of Pangea, Gondwana, and Laurasia.

These are characterized by functional lungs, a caudal fin that confluent with the dorsal and anal fins, pectoral and pelvic fins flipperlike, large scales, and larvae without external gills (Nelson 2006). The air bladder (lung) of all extant forms is unpaired. The extant species are all freshwater forms. There is one species of Australian lungfishes (family Ceratodontidae), one species of South American lungfishes (family Lepidosirenidae; foundin Brazil and Paraguay), and four species of African lungfishes (Family Protopteridae; all placed in the genus Protopterus).

There are various taxonomic schemes for the class Sarcopterygii. Nelson (2006) recognizes two subclasses of Sarcopterygii: Coelacanthimorpha and Dipnotetrapodomorpha. Coelacanthimorpha includes one order, Coelacanthiformes (coelacanths), with eight families that include only fossil members and one family that includes two living species of coelacanth as well as fossil members. The subclass Dipnotetrapodomopha is a new term that includes several orders and families, with the order Ceratodontiformes including the three extant families, three extant genera, and six modern species.

Anatomy and morphology

The lungfishes (dipnoi) have platelike teeth useful for crushing and grinding (Nelson 2006). The six extant species of lungfishes are placed by Nelson in the order Ceratodontiformes of the superorder Ceratodontimorpha. These are characterized by functional lungs, a caudal fin that confluent with the dorsal and anal fins, pectoral and pelvic fins flipperlike, large scales, and larvae without external gills (Nelson 2006). The air bladder (lung) of all extant forms is unpaired. The extant species are all freshwater forms. There is one species of Australian lungfishes (family Ceratodontidae), one species of South American lungfishes (family Lepidosirenidae; foundin Brazil and Paraguay), and four species of African lungfishes (Family Protopteridae; all placed in the genus Protopterus).

All lungfish demonstrate an uninterrupted cartilaginous notochord and an extensively developed palatal dentition. The lungfish is a true carnivore. Basal lungfish groups may retain marginal teeth and an ossified braincase, but derived lungfish taxa, including all modern species, show a significant reduction in marginal bones and a cartilaginous braincase. The bones of the skull roof in primitive lungfish are covered in a mineralized tissue called cosmine, but in post-devonian lungfishes, the skull roof is subdermal and the cosmine covering is lost. All modern lungfish show significant reductions and fusions of the bones of the skull roof, and the specific bones of the skull roof show no homology to the skull roof bones of actinopterygiians or tetrapods.

The dentition of lungfish is conspicuously different from that of any other vertebrate group. Odontodes on the palate and lower jaws develop in a series of rows to form a fan-shaped occlusion surface. These odontodes then wear to form a uniform crushing surface. In several groups, including the modern lepidosireniformes, these ridges have been modified to form occluding blades.

The modern lungfishes have a number of larval features, which suggest paedomorphosis. They also demonstrate the largest genome among the vertebrates.

Modern lungfish all have an elongate body with fleshy paired pectoral and pelvic fins and a single unpaired caudal fin replacing the dorsal, caudal, and anal fin of most fishes.

Ecology and life history

Behavior

African and South American lungfish are capable of surviving seasonal desiccation of habitats by burrowing into mud and estivating throughout the dry season. Changes in physiology allow the lungfish to slow its metabolism to as little as 1/60th of the normal metabolic rate, and protein waste is converted from ammonia to less-toxic urea (normally, lungfish excrete nitrogenous waste as ammonia directly into the water). Burrowing is seen in at least one group of fossil lungfish, the Gnathorhizidae. It has been proposed both that burrowing is plesiomorphic for lungfish as well as that gnathorhizids are directly ancestral to modern Lepidosireniformes, but it is possible that the similarity is simply due to convergent evolution or parallel evolution. Lungfish can be extremely long-lived. The Queensland species lungfish at the Shedd Aquarium in Chicago has been part of the permanent live collection since 1933.

Taxonomy

The relationship of lungfishes to the rest of Osteichthyes is well-understood:

• Lungfishes clade closest with Powichthyes, and then with the Porolepiformes.
• Together, these taxa form the Dipnomorpha, the sister group to the Tetrapodomorpha.
• Together, these form the Rhipidistia, the sister group to the coelacanths.

The relationship 'among' lungfishes to each other is significantly more difficult to resolve. While Devonian lungfish retain enough ossification of the endocranium to determine relationships, post-Devonian lungfish are represented entirely by skull roofs and teeth, as the rest of the skull is cartilaginous. Additionally, many of the taxa that have been identified may not be monophyletic. Current phylogenetic studies support the following relationships of major lungfish taxa:

Class Osteichthyes
Subclass Sarcopterygii
Order Dipnoi

,—†Family Diabolichthyidae
| ,—†Family Uranolophidae
| |  __,—†Family Speonesydrionidae
'-|-|  '—†Family Dipnorhynchidae
    |     ,—†Family Stomiahykidae
    '----|___ ,—†Family Chirodipteridae
          |      '-|—†Family Holodontidae
          |------†Family Dipteridae
          |  __,—†Family Fleurantiidae
          '-|  '—†Family Rhynchodipteridae
              '—†Family Phaneropleuridae
                     | ,—†Family Ctenodontidae
                       '-| ,—†Family Sagenodontidae
                          '-|—†Family Gnathorhizidae
                             '—Order Ceratodontiformes
                                  |—†family Asiatoceratodontidae
                                  |—†Family Ptychoceratodontidae
                                  |—Family Ceratodontidae
                                  |  '—†Genus Ceratodus
                                  |  '—†Genus Metaceratodus
                                   '—Family Neoceratodontidae
                                        | '—†Genus Mioceratodus
                                        | '—Genus Neoceratodus - Queensland lungfish
                                        '—Order Lepidosireniformes
                                               '—Family Lepidosirenidae - South American lungfish 
                                               '—Family Protopteridae - African lungfish

The following taxonomy is drawn from Nelson (2006).

• Class SARCOPTERYGII

• Subclass Coelacanthimorpha

Order Coelacanthiformes (coelacanths)

Family Miguashaiidae †

Family Diplocericidae †

Family Hadronectoridae †

Family Rhabdodermatidae †

Family Laugiidae †

Family Whiteiidae †

Family Caelacanthidae †

Family Mawsoniidae †

Family Latimeriidae (coelacanths or gombessas)

Genus Latimeria

Species Latimeria chalumnae

Species Latimeria menadoesnis

Genus Holophagus †

Genus Libys †

Genus Macropoma †

Genus Undina †

• Subclass Dipnotetrapodomorpha

• Unranked 1a. Onychodontida

Order Onychodontiformes †

• Unranked 1b. Rhipidistia

• Unranked 2a. Dipnomorpha

• Superorder Porolepimorpha †

Order Porolepiformes †

Dipnoi (Lungfishes)

• Superorder Dipterimorpha †

Family Diabolepididae †

Family Uranolophidae †

Family Dipnorhynchidae †

Family Chirodipteridae †

Family Stomiahykidae †

Family Dipteridae †

Family Rhynchodipteridae †

Family Fleurantiidae †

Family Phaneropleuridae †

Family Ctenodontidae †

Family Fleurantiidae †

• Superorder Ceratodontimorpha

Order Ceratodontiformes (living lungfishes)

Suborder Ceratodontoidei

Family Ceratodontidae (Australian lungfishes)

Genus Neoceratodus

Species Neoceratodus forsteri

Suborder Lepidosirenoidei

Family Lepidosirenidae (South American lungfishes)

Genus Lepidosiren

Species Lepidosiren paradoxa'

Family Protopteridae (African lungfishes)

Genus Protopterus

Species Protopterus aethiopicus

Species Protopterus amphibius

Species Protopterus annectens

Species Protopterus dolloi

• Unranked 2b. Tetrapodomorpha

• Unranked 3a. Rhizodontimorpha (Rhizodontida) †

Order Rhizodontiformes †

Family Rhizodontidae †

• Unranked 3b. Osteolepidimorpha †

• Unranked 4a. Unnamed Ostelepidiformes and Elipistostegalia and Tetrapoda

Order Ostelepidiformes †

• Unranked 4b. Unnamed Elipistostegalia + Tetrapoda

• Infraclass Elipistostegalia †
• Tetrapoda (tetrapods)

References

ISBN links support NWE through referral fees

• Ahlberg, PE, Smith, MM, and Johanson, Z, (2006). Developmental plasticity and disparity in early dipnoan (lungfish) dentitions. Evolution and Development 8(4):331-349.
• Palmer, Douglas, Ed. The Simon & Schuster Encyclopedia of Dinosaurs & Prehistoric Cretures. A Visual Who's Who of Prehistoric Life. Pg. 45. Great Britain: Marshall Editions Developments Limited. 1999.
• Schultze, HP, and Chorn, J., (1997). The Permo-Herbivorus genus Sagenodus and the beginning of modern lungfish. Contributions to Zoology 61(7):9-70.

• Nelson, J. S. 2006. Fishes of the World, 4th edition. Hoboken, NJ: John Wiley & Sons. ISBN 0471250317.

• Rosen, D. E., P. I. Forey, B. G. Gardiner, and C. Patterson. 1981. Lungfishes, tetrapods, paleontology, and plesiomorphy. Bull. Am. Mus. Nat. Hist. 167(4): 159-276.

External links

Wikispecies has information related to:

Dipnoi

Wikibooks Dichotomous Key has more about this subject:

Dipnoi

• Dr Anne Kemps - Lungfish Information site
• Lungfish information site
• Dipnoiformes at Palaeos.com
• Dipnoi at the University of California Museum of Paleontology
• Tree of life illustration showing lungfish's relation to other organisms

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